This invention relates to a power supply used to provide current for the resistive magnet of magnetic resonance imager in such a way that the magnetic field produced by the magnet exhibits high stability.
Magnetic resonance imaging (MRI) is a widely used diagnostic imaging method, which gives superior results compared with other modalities, such as x-rays or ultrasound. Its only drawback is the high cost of the equipment, typically 5-20 million Finnish Marks (.about.1-4 million US$). Thus, it is important to invent a way of decreasing the price.
The most common type of magnet (for MRI) is the superconducting one, which gives the highest field strengths, 0.5-2 Tesla, but which are expensive to produce and to use. The next most common one is the permanent type, which typically gives a field of 0.2-0.3 Tesla. The least used type is the resistive one, because it uses a lot of electrical power if a high field strength is to be obtained.
At lower fields, 0.1-0.2 T, the resistive magnet is the most economic solution, but its use has been limited because of the high requirements placed in MRI on the field stability, which has been difficult to fulfil with resistive magnets, which can be designed either with an air core or with an iron core.
To keep the quality of the MRI image unaffected the main magnetic field of the scanner has to be stable within 0.5-0.1 ppm (parts per million) during the imaging time (1-10 min). To obtain such a good field the current in the magnet has to be carefully controlled. In addition, external field disturbances must be attenuated.
The field strength is proportional to the magnet current. The temperature of the magnet wire can change several tenths of degrees, and this requires the power supply to work at a wide range of voltages. The field is further affected by the physical dimensions of the magnet, which may change with the temperature. This effect has to be compensated. The amount of compensation can be based on knowledge of the temperature, obtained by measuring it, or, alternatively, on a direct field measurement.
Monitoring and regulating the current within the required 0.5-0.1 ppm is difficult but possible. It can be done for example by measuring the voltage across a high quality resistor in series with the magnet. Another known method is to use a so called current transformer, as described in U.S. Pat. No. 4,616,174.
If external fields, varying with the time in the frequency range of 0.01-150 Hz, are superposed on the MRI field they will induce artifacts into the image. Superconducting magnets tend to keep their magnetic flux constant, and in this way attenuate external fields. A similar effect can be obtained by shorting the terminals of a resistive magnet for voltages in the said frequency range. This is, however, effective only for frequencies which are large compared with the L/R time constant of the magnet which typically has an L of about 1 Henry and an R of 1 Ohm and thus a time constant of about 1 second. This kind of attenuation cannot be obtained from a permanent magnet.
An additional requirement on the magnet power supply comes from the so called gradient fields. Gradient fields are fast changing fields which are added to the main field during imaging and induce voltage spikes into the magnet electrical circuit. These spikes can have magnitudes of several volts and a frequency content in the range of 1-2000 Hz. If the magnet power supply has a high output impedance at these frequencies it must also have a high dynamic range so as not to become nonlinear during the spikes. A low impedance supply, on the other hand, would produce a current surge during the voltage spike, which would disappear with a time constant L/(R+R.sub.out), where L is the magnet inductance, R the magnet resistance, and R.sub.out the output resistance of the power supply.
Other required properties of the power supply are a good efficiency, which obviates the use of water cooling, and a low value for radiated electromagnetic interference. It is also desirable that the supply forms a clean load on the mains supply, meaning that it should include a so called cos.o slashed. correction.
This application discloses a power supply, which fulfills all the presented requirements for a resistive MRI magnet. The invention is based on tailoring the desired shape on the output impedance by the use of two separate feedback loops.